The present invention relates to electrically operated vacuum regulator valves employed for controlling purge flow of fuel vapors from a storage device, typically a charcoal filled canister, employed to trap fuel vapors from a fuel tank. Such valves are employed on current production light motor vehicles for controlling the emissions of evaporative hydrocarbons into the atmosphere and are referred to as Vapor Management Valves (VMV).
An example of an electrically operated VMV employing a solenoid actuated atmospheric bleed valve (EVR) for controlling the vacuum signal pressure on one side of a diaphragm operated regulator valve is shown and described in U.S. Pat. No. 5,277,167. The aforesaid known valve has a preload spring biasing the diaphragm operated regulator valve to the closed position such that a predetermined differential pressure across the diaphragm is required to cause the valve to open. In such an arrangement as the aforementioned known valve, the preload spring is set on the diaphragm to provide the desired amount of valve opening in response to certain predetermined levels of a manifold vacuum or depression during engine operation applied to control pressure chamber. The manifold vacuum applied to one side of the regulator valve diaphragm may be attenuated by electrically controlling the EVR solenoid valve to bleed atmospheric air into the vacuum signal chamber to modify or alter the vacuum in the control chamber in accordance with a pre-selected program of vapor purge suitable for various engine operating conditions, e.g., RPM and load. However, for a predetermined calibration of the preload spring for desired opening at a pre-selected level of manifold vacuum, the force of the preload spring is not sufficient to hold the regulator valve closed when the engine is shut off in the event that an abnormally high pressure is built up in the canister and applied to the opposite side of the diaphragm operating the regulator valve. Thus, the regulator valve diaphragm is lifted permitting undesired flow of fuel vapor from the canister to flow through the regulator valve and into the engine inlet and consequently to the atmosphere when the engine is not operating.
In the aforesaid known electrically operated VMV, the differential pressure across the regulator valve diaphragm required to open the regulator valve to permit canister vapor to enter the engine inlet is typically set at a value which corresponds to a desired level of engine manifold vacuum during engine operation. This is accomplished by the calibration of the diaphragm preload spring with a known vacuum signal provided at the inlet to the pressure signal chamber.
Thus, it has been desired to provide a way or means of preventing an electrically operated VMV from bleeding fuel vapor from the canister to the engine air inlet when the engine is not operating for an VMV valve that is calibrated to provide a desired flow in accordance with pre-selected level of engine manifold vacuum when the engine is operating. It has been particularly desired to provide a way or means of accomplishing this result without the need for costly redesign and retooling of an existing high volume production VMV.